Environmentally sustainable energy production and reduction of greenhouse gas emissions can be achieved by use of carbon-neutral energy sources. Wind, geothermal, solar, hydroelectric, and biofuels are now being developed as sustainable sources of domestic energy production. Unlike wind, solar, or hydroelectric energy sources, biofuels can be converted into energy-dense, liquid fuels that are compatible with current energy distribution and consumption systems. Since biofuels have the potential to reduce global carbon emission, they are an attractive part of the mix of sustainable energy solutions. In addition, oil-based biofuels are one of the few renewable, energy-dense fuels that can be used by the aviation and shipping transportation sectors. However, demands for food and environmental sustainability limit the use of crops or plants for biofuel production.
Unicellular algae are a prime candidate for production of biofuels. In contrast to plants, unicellular algae do not partition large amounts of biomass into supportive structures such as stems and roots. Under near ideal growth conditions algae direct most of their energy into cell division (6-12 hour cycle), allowing for rapid biomass accumulation. Under stress conditions (e.g., low nitrogen) or in the presence of exogenous reductants (sugar, glycerol) metabolism is redirected towards the production of energy-dense storage compounds such as lipids. Many unicellular algae are facultatively capable of producing 4% to 60% lipids per gram dry weight under the appropriate growth conditions (e.g., stress or photoheterotrophic growth), making them one of the most efficient biofuel production systems known. It has been estimated that, on area basis, algae may produce up to twenty times the fuel of any land plant system. Despite these useful characteristics, many challenges remain for developing efficient, large-scale production of biofuel from algae.